High density connector having a ball type of contact surface

ABSTRACT

A connector for use with a circuit substrate having a plurality of elongated conductors. A solder ball is fused to the end of the conductors for connection of the connector to a circuit substrate. Method for attaching the solder ball to the end of the conductors using a solder paste with or without a preformed solder ball. The conductors are fitted into passages of an interface member. A tail end of each conductor terminates in a well in a surface of the interface member that facilitates the attachment of solder ball to the end of the conductors. The passages in the interface member hold the conductor tail ends in place while providing a clearance around the conductor to accommodate the effects of thermal expansion and contraction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.08/851,165, filed May 02, 1997 and now U.S. Pat. No. 6,139,336, whichclaims the benefit of U.S. Provisional Patent application Ser. No.60/030,799, filed Nov. 14, 1996 and entitled “BALL GRID ARRAY HIGHDENSITY CONNECTOR,” both of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrical connectors and moreparticularly to high I/O density connectors such as connectors that areattachable to a circuit substrate by use of a solder ball contactsurface.

2. Brief Description of Earlier Developments

The drive to reduce the size of electronic equipment, particularlypersonal portable devices, and to add additional functions to suchequipment has resulted in an ongoing drive for miniaturization of allcomponents, especially electrical connectors. Efforts to miniaturizeelectrical connectors have included reductions in the pitch betweenterminals in single or double row linear connectors, so that arelatively high number of I/O or other lines can be interconnected byconnectors that fit within the tightly circumscribed areas on circuitsubstrates that are allotted for receiving connectors. The drive forminiaturization has also been accompanied by a shift in manufacturingpreference to surface mount techniques (SMT) for mounting components oncircuit substrates. The confluence of the increasing use of SMT and therequired fine pitch of linear connectors has resulted in approaching thehigh volume, low cost limits of SMT for mounting connectors that employpresently available mounting designs. The limit is being reached becausefurther reductions in pitch of the terminals greatly increase the riskof bridging adjacent solder pads or terminals during reflow of thesolder paste. Array electrical connectors have been proposed to satisfythe need for increased I/O density. Such electrical connectors have atwo dimensional array of terminal tails and can provide improveddensity. However, these connectors present certain difficulties withrespect to attachment to the circuit substrate by SMT techniques becausethe surface mount tails of most, if not all, of the terminals must beattached beneath the connector body. As a result, the mountingtechniques used must be highly reliable because of the difficulty invisually inspecting the solder connections and repairing them, iffaulty.

Mounting techniques for other electronic components have addressed thereliability of solder connections in hard to inspect positions. Forexample, integrated circuit (IC) mounting to plastic or ceramicsubstrates have increasingly employed solder balls and other similarpackages to provide such a reliable attachment. In a solder balltechnique, spherical solder balls attached to the IC package arepositioned on electrical contact pads of a circuit substrate to which alayer of solder paste has been applied, typically by use of a screen ormask. The unit is then heated to a temperature at which the solder pasteand at least a portion of the solder ball melt and fuse to an underlyingconductive pad formed on the circuit substrate. This heating process iscommonly referred to as solder reflow. The IC is thereby connected tothe substrate without need of external leads on the IC.

While the use of solder balls and similar systems in connecting ICs to asubstrate has many advantages, a corresponding means for mounting anelectrical connector or similar component on a circuit substrate hasrecently become desirable. The use of such techniques in mountingelectrical connectors has lagged the use in mounting ICs because the useof solder ball technologies in mounting an electrical connector orsimilar component to a circuit substrate presents complexities notencountered with IC mounting. For example, ICs that have employed solderballs generally present a flat attachment surface. By contrast,connectors usually do not present a flat attachment surface but ratherpresent a series of elongated conductors, commonly referred to asterminal tail ends. Attachment of a solder ball to the small end surfacepresented by the tip of a terminal tail end presents manufacturingdifficulties not present in the attachment of solder balls to a flatsurface.

In addition to the manufacturing difficulties, connectors are generallymore susceptible to solder joint stress due to the effects ofdifferential Coefficients of Thermal Expansion (CTE) between theconnector and the circuit substrate. This susceptibility is primarilydue to size and geometry differences between a connector and an IC. Forexample, IC mounting surfaces are generally on the order of 2.5centimeters square. Connector mounting surfaces, on the other hand,generally have a narrow width (e.g., 0.5 centimeters or less)and a muchlonger length (e.g. 5.0 centimeters or more). Primarily because of therelatively long length of the connector, differences in CTE between aconnector and a circuit substrate potentially have a much greater effecton the solder joints than the differences in CTE between an IC and acircuit substrate.

Connectors attached to circuit substrates via solder ball techniques arealso more susceptible to joint stress than a conventional SMT attachmenttechnique. For example, a conventional SMT attaches connector terminaltails to a circuit substrate horizontally, providing more attachmentsurface area for the solder joint. The additional surface area of thesolder joint in the conventional SMT technique is stronger and,consequently, more tolerant of differences in CTE between the connector,terminal tails and circuit substrate. A solder ball connection, on theother hand, attaches a connector terminal tail vertically to the circuitsubstrate with the end of the terminal tail directly mated to thecircuit substrate, reducing the amount of attachment surface area. As aresult of the smaller attachment surface, differences in CTE are muchmore likely to stress the terminal tail to circuit substrate jointresulting in failure or quality problems.

Furthermore, in most circuit substrate applications, the electricalcomponent mounting surfaces of the surface mount connections must meetstrict coplanarity requirements. Thus, the use of solder balls to attacha connector to a circuit substrate imposes the requirement that thesolder balls are coplanar in order to ensure a substantially flatmounting interface. So that, in the final application the balls willreflow and solder evenly to a planar circuit substrate. Any significantdifferences in solder coplanarity on a given mounting connection cancause poor soldering performance when the connector is reflowed onto aprinted circuit board. Accordingly, users specify very tight coplanarityrequirements to achieve high soldering reliability, on the order of 0.1to 0.2 mm for example. By providing a connection using a solder balltechnique, the coplanarity requirements can be met and sometimesexceeded. Unlike conventional SMT, the solder ball can absorb variationsin terminal tail length by changing shape upon the application of heatto bridge the gaps between the terminal tail ends and the circuitsubstrate connections.

The present invention recognizes that there is a need for an improvedelectrical connector apparatus and accompanying electrical connectorconstruction techniques that address the shortcomings of presentelectrical connectors.

SUMMARY OF THE INVENTION

The invention meets the above needs by providing an improved electricalconnector for use in forming an electrical connection between a contactportion of an electrical component and a contact portion of a circuitsubstrate and method for constructing the electrical connector. Theelectrical connector comprises a connector body, a plurality ofelectrical contacts disposed on the connector body arranged toelectrically mate with the contact portion of the electrical component,a plurality of electrical elongated conductors, alternately referred toas terminal tails, disposed on the connector body are arranged to forman electrical connection with the circuit substrate. The elongatedconductors are in electrical communication with corresponding electricalcontacts. A substrate contact, such as a solder ball, is connected via abutt joint on an end of each of the elongated conductors such that anelectrical connection between the elongated conductors and the contactportion of the circuit substrate is selectively accommodated.

Each of the elongated conductors is disposed in a passage that hascross-sectional diameter a little larger than the cross-sectionaldiameter of the elongated conductor. As a result, clearance is providedbetween the sides of the electrical conductor and the side walls of thepassage. Preferably, the cross-section is substantially rectangular inshape. The passages terminate in wells that are disposed across a planarface of the connector. The wells have a rectangular top opening that islonger along a length of the connector face. Moreover, a portion of theconnector body proximate the elongated connector tail end is coated withan antimigration solution such as oleophobic-hydrophobic flourochemicalpolymer to assist the process of solder ball formation and attachment.

One of the primary manufacturing challenges in manufacturing the abovedescribed connector involves the method of fusing substrate contactmaterial (e.g., a solder ball) to the end of the tail portion of theelongated conductors. The invention accomplishes this attachment task byfirst forming a well, as described above, within a planar surface of theconductor. Here, the planar surface is provided by an interface memberthat can be formed separately and attached to the body of the connectoror alternatively formed as an integral component with the body. Ineither case, the tail ends of the elongated conductors are inserted intopassages formed in the interface member such that the tail endsterminate within a predefined range of the mounting surface and areexposed within the well. The well is then filled with a predeterminedamount of solder paste. Finally, the substrate contacts are fused to theends of the elongated conductors according to two embodiments.

In the first embodiment, a premanufactured substrate contact member suchas a solder ball is seated into the paste. The tail end, substratecontact member and solder paste are then heated to a predefinedtemperature above the melting point of the paste such that the solderpaste coalesces around the substrate contact member and joins it to theelongated conductor end.

According to a second embodiment, no premanufactured substrate contactmember is used. Rather, a pre-specified amount of solder paste isapplied to the well. Thereafter, the tail end and solder paste areheated to a predefined temperature, above the melting point of thesolder paste. As a result, the solder paste coalesces into a ballattached to the end of the elongated conductor.

The process for forming a substrate contact on an elongated conductor asdescribed above is further enhanced by coating the well with ananti-migration solution such as oleophobic-hydrophobic fluoropolymer.Thereafter, when the solder paste is heated, the paste is repelled fromthe treated surfaces of the well and interface member. This results in amore uniform ball formation. The substrate contact attachment processcan be further enhanced by passivating a portion of the tail end suchthat solder paste will not attach to the passivated portion. As aresult, the solder can attach only to the very end of the elongatedconductor. Solder flow restriction at the tail end can be enhanced bypassivating the tail end, coating the tail end with an anti-migrationsolution or both.

BRIEF DESCRIPTION OF THE DRAWINGS

Other uses and advantages of the present invention will become apparentto those skilled in the art upon reference to the specification and thedrawings, in which:

FIG. 1 is a top plan view of a card edge connector which represents apreferred embodiment of the connector of the present invention;

FIG. 2 is a front view of the card edge connector shown in FIG. 1;

FIG. 3 is a side view of the card edge connector shown in FIG. 1;

FIG. 4 is an cross-sectional view through 4—4 in FIG. 1;

FIG. 5 is a detailed view of the substrate contact area of FIG. 4;

FIG. 6 is a schematic cross-sectional view of completed substratecontact connection to a first tail end embodiment;

FIG. 7 is a schematic cross-sectional view of a completed substratecontact connection to a first tail end embodiment;

FIG. 8 is a bottom plan view of a substrate contact connection;

FIG. 9 is a schematic cross-sectional view of a substrate contactconnection according to a first method;

FIG. 10 is a schematic cross-sectional view of a substrate contactconnection according to a second method; and

FIG. 11 is a graph comparing relative terminal tail height versussubstrate contact height.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to presently preferred embodiments, a linear connector havinga solder ball type electrical contact surface and methods for attachingthe solder balls to the connector will now be described with referenceto the Figures. It will be appreciated by those of ordinary skill in theart that the description given herein with respect to those Figures isfor exemplary purposes only and is not intended in any way to limit thescope of the invention. For example, an electrical connector isdescribed herein having a substantially rectangular mounting surfacewherein the length of the mounting surface is substantially greater thanits width. However, the particular dimensions described herein withreference to that connector are merely for the purpose of illustrationand are not intended to be limiting. The concepts disclosed herein havea broader application to a much wider variation of connector mountingsurface geometries. The concepts disclosed with reference to thisconnector could be employed, for example, with a connector having aconnection mounting surface having a more square or radial geometry.

Referring now to FIGS. 1-3, there is shown top, front and side views,respectively, of an electrical connector 1 of the present invention. Asshown, connector 1 comprises a connector body 5, a plurality ofelongated conductors 8, an interface member 10, electrical elementcontacts 9, hold down element 13 with an upwardly extending arm 15 andconnector latches 24. As best shown in FIG. 2, the front of connector 1comprises a plurality of electrical contacts 9 arranged within twoopenings, which are designed to selectively engage and mate withcorresponding electrical contacts for an electrical element such asthose electrical contacts present on a daughter board. As is describedin further detail below, each electrical contact 9 is in electricalcommunication with a corresponding elongated conductor 8 such that acurrent flow path is established therebetween. Elongated conductors 8are designed for permanent connection to electrical contacts on acircuit substrate (not shown). Thereafter, electrical communication canbe selectively established between an electrical element inserted intoelectrical connector 1 and a circuit substrate to which it is attached.In this way, for example, electrical connector 1 can be connected to acircuit substrate such as a card edge connector of the type used in acomputer motherboard. Thereafter, a user, via attached electricalconnector 1, can selectively add a daughter board or boards (e.g.,memory modules) to the computer motherboard.

The electrical contacts of connector 1 comprise a linear array stackedin two parallel rows 36A and 36B, each row accepting a single daughterboard. A user inserts an electrical element laterally into connector 1.As each daughter board is inserted, connector latches 24 that correspondto the particular row, 36A or 36B, engage and lock the electricalelement into place on connector 1. Although two parallel rows are shownwith respect to connector 1, the concepts disclosed herein are equallyapplicable to a connector having one row, three rows and so on.Preferably, connector body 5 and connector latches 24 are constructedfrom a molded plastic material to reduce weight.

Further details of the arrangement of connector 1 can be seen withreference to FIG. 4. FIGS. 4 and 5, presents a cross-sectional view ofconnector 1 taken along the line 4—4. As shown therein, connector 1further comprises stacked insert molded wafers 2, which are lockedtogether in a pin and socket arrangement 4. Thus locked together, wafers2 are retained in housing 5 by a series of dimples 6 which are fittedinto cylindrical openings 7. A plurality of elongated conductors 8extend into each of wafers 2. Each elongated conductor therein joins inelectrical communication with a corresponding electrical contact 9. Eachelongated conductor 8 also extends out from wafer 2 and extends downinto interface member 10. A bend 29, preferably about 90° in thedisclosed embodiment, along with the length of elongated conductor 8allows connector 1 to provide interfaces on two different planes. Forexample, the bend allows connector 1 to interface vertically withrespect to the board, while interfacing with an electrical element thatis positioned horizontally with respect to the board. As such, theelectrical element projects horizontally over the circuit substratesurface conserving circuit substrate height. Other degrees of bends 29could be used as required to adjust to a variety of applicationgeometries without departing from the inventive aspects of the presentinvention. For example, the elongated conductors could have no bend, inwhich case, the electrical element would project vertically with respectto the circuit substrate. The tail end of each elongated conductor 8extends through a corresponding passage 25 in interface member 10 and isterminated by a substrate contact 12 such as a solder ball. Eachelongated conductor 8 preferably has a substantially rectangularcross-sectional shape such as a square; however, other cross-sectionalshapes such as round may function equally as well.

FIG. 5 presents a more detailed view of elongated conductors 8 inrelation to interface member 10. Interface member 10 can be a separatecomponent that is attached to connector body 5 or, alternatively, couldbe integrally formed therewith. As shown, interface member 10 has aplurality of passages 25, one passage 25 being provided to guide andsupport one elongated conductor 8 with a tail end of each elongatedconductor 8 disposed within a passage 25. Significantly, the diameter ofpassage 25 is a little larger than the diameter of elongated conductor8. Accordingly, the tail end of elongated conductor 8 is disposed withinpassage 25 such that clearance 18 is provided between the sides ofelongated conductor 8 and the walls of passage 25. For example, whereaselongated conductor 8 has a width of about 0.305 mm, passage 25 has awidth of about 0.38 mm. Clearance 18 provides an important feature intolerating differences in Coefficients of Thermal Expansion (CTE)between the material forming the interface member 10 and the materialforming the circuit substrate on which the connector is mounted. Thatis, passages 25 are sized to provide guidance to elongated conductors 8during substrate contact 12 attachment and further during connector 1attachment to a circuit substrate. However, elongated conductors 8 havea clearance with and are not physically connected to interface member10, permitting interface member 10 to expand and contract without theside walls of the passages impinging upon elongated conductors 8 becauseof the clearance. The amount of clearance is related to the differentialin CTE noted above. As a result, stress is reduced in a solder jointbetween elongated conductors 8 and the circuit substrate during thermalexpansion and contraction cycles. Passage 25 preferably comprises across-section substantially similar to the cross-section of elongatedconductor 8. For example, if elongated conductor 8 has a substantiallyrectangular cross-section then passage 25 also has a substantiallyrectangular cross-section. Each passage 25 terminates in the bottom ofwell 11 and comprises a wide inlet cavity 27 that provides guidance toelongated conductor 8 during insertion into interface member 10. Eachend of a conductor 8 terminates within well 11 and therein is fused to asubstrate contact 12, which is also partially disposed within well 11.For a variety of reasons, including the bend in elongated conductors 8,the termination height of the elongated conductors 8 within wells 11will vary among conductors 8. This height variation is commonly referredto as coplanarity.

Referring now to FIG. 8, a bottom plan view of a substrate contact 12 inrelation to well 11 is depicted. As shown, well 11 is substantiallyrectangular in cross-section having a length 1 slightly larger than awidth w. For example, width w is preferably 0.5 mm; whereas, length 1 ispreferably 0.55 mm. Additionally, according to a first embodiment,substrate contact 12 is on the order of 0.3 mm to 0.5 mm. Thus, there isa clearance of at least about 0.05 mm between substrate contact 12 andtwo sides of well 11. As with passages 25, the rectangular shape of well11 also accommodates effects of thermal expansion and contraction. Theclearance between the sides of well 11 and substrate contact 12 ensuresthat well 11 will not impinge upon the solder joint during expansion andcontraction.

The length of well 11 is aligned with the length of interface member 10to accommodate thermal cycles while maximizing conductor 8 density.Because interface member 10 is longer along its length than its width,the effects of expansion and contraction will be correspondingly greateralong the length of interface member 10 than along the width.Accordingly, a top opening of well 10 has dimensions that accommodatethe lengthwise thermal expansion and contraction. Along the width ofinterface member 10, the effects of expansion and contraction are lessof an issue because of the smaller size relative to the length.Consequently, the width of well 11 can be narrower than the length. Theoverall result is a well 11 that is sized to accommodate expansion andcontraction along the length of connector 1 while allowing a higherdensity of wells 11 and conductors 8 along the width of connector 1.Additionally, because the top opening of well 11 is dimensioned in thisway, tighter elongated conductor 8 densities across the width ofinterface member 10 are possible while allowing well 11 to maintain arequired volume of solder paste during substrate contact attachment.

Further details of the connection of substrate contacts 12 to elongatedconnectors 8 will now be described with reference to FIGS. 6-9.Referring now to FIG. 6, an embodiment of the attachment of substratecontact 12 to end of the elongated conductor 8 is shown. The substratecontact 12 is fused to the end of elongated conductor 8 with a buttjoint. To ensure a quality butt joint between substrate contacts 12 toelongated conductors 8, a portion of the tail end of each elongatedconductor 8 has a passivated surface 17. Passivated surface 17 ensuresthat solder, used during the attachment process, does not wick along thesides of elongated conductor 8 and potentially enter clearance 18 duringsubstrate contact reflow. As described above, clearance 18 accommodatesdifferences in Coefficients of Thermal Expansion. Accordingly, if solderenters clearance 18, the reliability of a corresponding solder joint isjeopardized as interface member 10 expands and contracts. Therefore, ananti-solder wicking or non-solder wettable material is applied to thesurface 17. One preferred material for this purpose is nickel plating.While not intending to be bound by any particular theory, it is believedthat the solder resistant feature of this nickel plated area resultsfrom or is enhanced by the oxidation of the nickel plating, for example,by laser oxidation, steam exposure or ambient air exposure. Other solderwick resistant materials are believed to be useable for this purpose,such as fluorine containing solder resist coatings. Alternatively, acombination of nickel and fluorine could be used.

Other embodiments are possible to prevent solder wicking. For example,FIG. 7 depicts a second embodiment of the substrate contact andelongated conductor 8 interface. In this embodiment, the end ofelongated conductor 8 is beveled. As a result of the bevel, solder filet21 is formed between substrate contact 12 and the end of elongatedconductor 8 during the attachment of substrate contact 12 to elongatedconductor 8. This second embodiment also reduces the potential forsolder to wick into the clearance 18 by capturing solder within a bevel.

In the embodiments previously described, the tail end of each elongatedconductor 8 is positioned in a well 11. Each well 11 is substantiallyuniform in size and shape and provides several features of importancewith respect to the present invention. Referring to FIG. 9, for example,each well 11 ensures that a highly uniform amount of solder paste 19 isreceived therein using a process such as a simple deposit and squeegeeoperation. Thus, the amount of solder available for securing eachsubstrate contact onto an end of elongated conductor 8 is substantiallyuniform. The wells 11 locate the position of each substrate contact inthe lateral X-Y directions prior to attachment of the substrate contactsin the Z direction with respect to the bottom surface of interfacemember 10 and the end of elongated conductor 8. After solder reflow andbest shown in FIG. 6, the solder contained in paste 19 in well 11increases substrate contact 12 size by area 16.

The size of area 16 is affected by the height of the tip of conductor 8within well 11. The amount of solder paste deposited in each well 11 isaltered by the variation in elongated conductor 8 height. For example, aconductor 8 tip that is higher in well 11 will displace more solderpaste; whereas, a conductor 8 tip that is lower in well 11 will displaceless solder paste. If less paste is available in the well, during reflowof the solder paste less paste is available to coalesce around thesubstrate contact, resulting in a slightly smaller substrate contact 12.A smaller substrate contact will result in a lower height. On the otherhand, when more paste is available in the well a larger substratecontact will result, corresponding to a higher substrate contact height.

The process above describes a substrate contact attachment via the useof preformed substrate contacts 12. However, according to a secondembodiment, the substrate contact interface can be formed without theuse of preformed substrate contacts. Referring now to FIG. 10 there isshown a method of substrate contact 12 attachment using only solderpaste. According to this method, a prespecified amount of solder pasteis deposited in and over well 11. This prespecified amount of solderpaste can be deposited by a commercial dispensing machine such as aCAM/A LOT 1818 available from Camelot Systems, Inc. After the solderpaste deposition, the connector is heated above the solder paste meltingpoint. The solder within the paste then coalesces into a substratecontact 12, which forms on the end of elongated conductor 8. Asdescribed above with reference to the preformed substrate contacttechnique, the variable volume of paste will effect the size andcoplanarity of the substrate contacts. Similar to the preformedsubstrate contact method, the displacement of solder paste by the heightof conductor 8 within well 11 will affect the final coplanarity in thismethod as well.

As previously mentioned, coplanarity of the substrate mounting face of aconnector utilizing substrate contact mounting is critical with any SMTdevice. In connector 1, there are two primary factors that influence thecoplanarity of the connector to circuit substrate interface: (1) thecoplanarity of the end surfaces, i.e., the tips, of elongated conductors8; and (2) the coplanarity of substrate contacts 12. The coplanarity ofthe tips of elongated conductors 8 is affected by a variety of factorssuch as length of the conductor 8, uniformity of the bends, the abilityto keep the conductors consistently parallel and the like. As a result,it is extremely difficult to maintain the conductor tips within thefinal coplanarity requirement of the circuit substrate manufacturerswhile maintaining high yields and low costs. As described more fullybelow, however, through the use of the substrate contact attachment inconnection with well 11, the restrictive coplanarity requirements forthe completed connector 1 is met while imposing a less restrictivecoplanarity requirement on the tips of elongated conductors 8.

Referring to FIG. 11, there is shown an exemplary graph of thecoplanarity of the tips of elongated conductors 8 before the attachmentof substrate contacts 12 (curve 32) versus the coplanarity of elongatedconductors 8 with the attached substrate contacts 12 (curve 34). Theunits along the abscissa represents different samples used in thecoplanarity measurement. The units along the ordinate represent unitsample height labeled 0, 1, 2 and so on; however, these units arelabeled to merely illustrate the relative relationship between the tipposition and corresponding substrate contact height. Actual units andvalues will vary depending on factors such as well dimensions andsubstrate contact size.

Curve 32 represents the height of an elongated conductor tip as measuredabove the bottom of well 11 (see also FIG. 6). Thus, at sample 0, forexample, the measured tip height is 0 units. At sample 1, the tip heightis about 0.75 and so on. At sample 5, the tip height is about 4 units.

Curve 34 represents the height above the bottom of well 11 of the sametip after attachment of the substrate contact 12 (see also FIG. 6). Forsample 0, the corresponding height after attachment of substrate contact12 is about 4.5 units. At sample 1, the corresponding substrate contactheight is about 5 units and so on. Finally, for sample 5, the substratecontact height is about than 6.5 units.

Comparing the curves 32 and 34 illustrates that the process of attachingsubstrate contact 12 to the end of conductor 8 absorbs some of thevariations in coplanarity of the tips of conductors 8. For example, insample 0, the difference between the tip height (preattachment curve 32)and the substrate contact height (post attachment curve 34) is about 4.5units. By contrast, at sample 5, the tip height is about 4 units,whereas the substrate contact height is about 6.5 units, for adifference of only 2.5 unit. Moreover, the total change in tip heightover the enter range of samples was about 4 units, but the total changeof substrate contact height over the same sample range was only about 2units.

To further illustrate the coplanarity regulation performed by the wellsconsider the following example. A typical connector before substratecontact attachment will have a number of elongated conductors 8 withdifferent tip heights. If, for example, one conductor 8 has a tip heightof 0 (i.e., the tip is exactly flush with the bottom of the well) andanother connector has a tip height of 4 units, the coplanarity betweenthe tips would be about 4 units. Under some circumstances thiscoplanarity variation of 4 units may be unacceptable. However, after theattachment of substrate contacts 12 according to the methods describedherein, the corresponding substrate contact heights will be 4.5 unitsand 6.5 units respectively, for a final coplanarity of about 2 units.Significantly, the final coplanarity is only 2 units versus 4 units.

In summary, the coplanarity differences of the tips of elongatedconductor 8 is counteracted during the attachment of substrate contacts12. Substrate contact size is transformed by the variable volume ofsolder paste placed in well 11 as a result of variations in heights ofelongated conductor tips. Consequently, the overall height of thesubstrate contact after attachment to the tip of conductors 8 issomewhat equalized. Accordingly, a connector 1 can be constructed with ahigher tolerance for elongated conductors coplanarity than would bepossible otherwise.

According to another aspect of the invention, the quality of substratecontact 12 attachment is enhanced by the application of ananti-migration or anti-wicking solution to an area in and around well11. Without the anti-migration solution, substrate contacts may formunevenly, sometimes wicking to the well edges. With the anti-migrationssolution applied, solder is repelled from the well edges and forms moreuniform substrate contacts on the ends of elongated conductors 8. Thepreferred anti-migration solution is oleophobic-hydrophobicfluoropolymer. Such a solution is commercial available from 3Mcorporation under the Fluorad brand name.

In the method of this invention, the substrate contact will preferablybe a solder ball. Those skilled in the art, however, will appreciatethat it may be possible to substitute other fusible materials which havea melting temperature less than the melting temperature of conductorbody 5 and elongated conductors 8. The fusible element can also have ashape other than a sphere. The ends of elongated conductors 8 willextend into well 11 by a sufficient amount to present adequate surfacearea for the substrate contact to fuse to, and will usually preferablyextend into the recess from about 25 percent to 75 percent and morepreferably to about 50 percent of the depth of the recess as previouslymentioned. The recesses ordinarily will be circular, square or the shapeof any other regular polygon in cross-section. When the conductiveelement is solder, it will preferably be an alloy which is in the rangeof about 3% Sn and 97% Pb to about 100% Sn and 0% Pb. More preferablythe alloy will be eutectic which is 63% Sn and 37% Pb and has a meltingpoint of 183° C. Typically, a “hard” solder alloy with a higher leadcontent would be used for mating to materials such as ceramics. The“hard” substrate contact will “mushroom” or deform slightly as itsoftens under typical SMT conditions, but will not melt . A “soft”eutectic ball is used for attachment to printed circuit boards and willusually reflow and reform itself under typical SMT conditions. Othersolders known to be suitable for electronic purposes are also believedto be acceptable for use in this method. Such solders include, withoutlimitation, electronically acceptable tin-antimony, tin-silver and leadsilver alloys and indium. Before the substrate contact or otherconductive element is positioned in a recess, that recess would usuallybe filled with solder paste.

Alternatively, in place of the substrate contact previously described, abody of material which is not fusible at SMT temperatures may beattached by reflow of the solder paste in the recesses onto thecontacts. The connector mounting interface would comprise a plurality ofinfusible spheres in a tightly coplanar array. Such a connector would besecured on a substrate by conventional SMT techniques.

While it is believed that a solder paste or cream incorporating anyconventional organic or inorganic solder flux may be adapted for use inthis method, a no clean solder paste or cream is preferred. Such solderpastes or creams would include a solder alloy in the form of a finepowder suspended in a suitable fluxing material. This powder willordinarily be an alloy and not a mixture of constituents. The ratio ofsolder to flux will ordinarily be high and in the range of 80%-95% byweight solder or approximately 50% by volume. A solder cream will beformed when the solder material is suspended in a rosin flux. Preferablythe rosin flux will be a white rosin or a low activity rosin flux,although for various purposes activated or superactivated rosins may beused. A solder paste will be formed when a solder alloy in the form of afine powder is suspended in an organic acid flux or an inorganic acidflux. Such organic acids may be selected from lactic, oleic, stearic,phthalic, citric or other similar acids. Such inorganic acids may beselected from hydrochloric, hydrofluoric and orthophosphoric acid. Creamor paste may be applied by brushing, screening, or extruding onto thesurface which may advantageously have been gradually preheated to ensuregood wetting.

Heating is preferably conducted in a panel infra red (IR) solder reflowconveyor oven. The connector would then be heated to a temperature abovethe melting point of the solder within the solder paste.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the present invention should not be limited to any singleembodiment, but rather construed in breadth and scope in accordance withthe recitation of the appended claims.

What is claimed is:
 1. A method of making an electrical connectorassembly comprising the steps of: providing an electrical connector witha housing and at least one contact, said contact having a tail extendingfrom said housing; providing an auxiliary housing having an openingtherethrough; placing a fusible material in said opening at a first sideof the auxiliary housing; inserting said tail into said opening; andfusing said fusible material to said tail, wherein after said fusiblematerial is fused to said tail, said fusible material extends out ofsaid opening at said first side for subsequent reflow fusing to anotherelectrical member positioned at said first side.
 2. The method asrecited in claim 1, further comprising the step of placing a formedelement in said fusible material and wherein said fusing step includesfusing said formed element to said tail.
 3. The method as recited inclaim 1, further comprising the step of maintaining said tail away froma side of said opening.
 4. The method as recited in claim 1, furthercomprising the step of preventing migration of said fusible material. 5.The method as recited in claim 4, wherein the preventing step comprisesthe step of treating one of said tail and said opening with ananti-migration solution.
 6. The method as recited in claim 1, whereinthe placing step comprises introducing a solder paste in said opening.7. A method of making an electrical connector assembly comprising thesteps of: providing an electrical connector with a housing and at leastone contact, said housing having an opening therethrough; placing afusible material in said opening; inserting a tail of said at least onecontact into said opening; fusing said fusible material to said tail;and preventing migration of said fusible material.
 8. The method asrecited in claim 7, wherein the preventing step comprises the step oftreating one of said tail and said opening with an anti-migrationsolution.
 9. A method of making an electrical connector comprising stepsof: providing a first subassembly comprising a first housing and atleast one electrical conductor connected to the first housing, saidconductor having an end; connecting a second housing to the firstsubassembly, wherein the conductor extends into an opening in the secondhousing, through a first side of the second housing, and into anenlarged well of the opening along a second different side of the secondhousing; placing fusible material in the enlarged well and on theconductor in the well; and fusing the fusible material to the end ofconductor, wherein after fusing the fusible material extends out of thewell for subsequent reflow fusing with another member placed at thesecond side of the second housing.
 10. The method as recited in claim 9,further comprising preventing migration of the fusible material.
 11. Themethod as recited in claim 10, wherein the preventing step comprisestreating the end of the conductor with an anti-migration solution. 12.The method as recited in claim 10 wherein the preventing step comprisestreating the opening with an anti-migration solution.
 13. The method asrecited in claim 11 wherein the step of inserting said tail into saidopening includes keeping a distal end of said tail within said auxiliaryhousing.
 14. The method as recited in claim 7 wherein the step ofproviding the electrical connector includes providing an auxiliaryhousing, said opening being located in said auxiliary housing.
 15. Themethod as recited in claim 7 wherein the step of inserting said tailinto said opening precedes the step of fusing said fusible material tosaid tail.